The present invention relates to a new and distinct cultivar of Helianthus decapetalus×multiflorus and will be referred to hereafter by its cultivar name, ‘Double Whammy’. ‘Double Whammy’ represents a new Helianthus, an herbaceous perennial grown for landscape use.
The Inventor discovered the new Helianthus as a naturally occurring whole plant mutation in 2009 in a nursery bed in Pukekohe, New Zealand. The parentage is unknown, however based on its characteristics, the new cultivar is an interspecific hybrid between Helianthus decapetalus and Helianthus multiflorus. 
Asexual propagation of the new cultivar was first accomplished by division by the Inventor in Mansell Gamage, Hereford, United Kingdom in spring of 2011. Asexual propagation by division and stem cuttings has shown that the characteristics of the new cultivar are stable and reproduced true to type in successive generations.